1. Field of the Invention
The present invention relates to a Flexible Printed Circuit (FPC), and more particularly, to an FPC for connecting a touch screen to a touch board with a touch IC mounted thereon and a liquid crystal display (LCD) device using the same.
2. Discussion of the Related Art
Among various types of flat panel display devices, LCD devices are being expanded in application field because the LCD devices are easily manufactured and have drivability of drivers, high image quality and a large screen.
Recently, instead of an input device such as a mouse or a keyboard which is conventionally applied to LCD devices, a touch screen that enables a user to directly input information with a finger or a pen is applied as an input device to LCD devices.
Especially, in portable terminals such as smart phones, LCD devices with a built-in touch screen in which the touch screen is built in a liquid crystal panel for slimming are developed.
FIGS. 1A and 1B are exemplary diagrams respectively illustrating general equivalent circuits of a touch screen. FIG. 1A illustrates an equivalent circuit. FIG. 1B shows a waveform of an output sensing signal (Rx signal) having noise (noise common voltage) “Vcom noise” caused by a common voltage when a driving signal (Tx signal) is applied to the touch screen so as to sense a touch.
A related art LCD device including a touch screen includes a panel including a touch screen 20; a main board (main PCB) on which a timing controller or a display driver IC (DDI) (hereinafter simply referred to as a controller), connected to an external system, for controlling the driving of gate lines and data lines inside a panel is mounted; a panel connection part for connecting the panel and the controller; a touch board (touch PCB) on which a touch screen driver IC 30 (hereinafter simply referred to as a touch IC) for driving the touch screen is mounted; and an FPC for connecting the touch screen and the touch IC.
In the LCD device including the touch screen, as illustrated in FIG. 1A, an equivalent circuit of the touch screen 20 includes a driving signal resistor RTX connected to the touch driver 10 that applies the driving signal (Tx signal) to the touch screen 20, a sensing signal resistor RRX connected to a touch sensing unit of the touch IC 30, and a plurality of capacitors CMutual, CTX and CRX which are connected to each other in series or parallel. Here, the touch driver 10 and the touch sensing unit may be included in the touch IC 30 included in the touch board.
In the related art LCD device including the touch screen, the touch driver 10 outputs the driving signal to a driving electrode of the touch screen 20 so as to sense a touch, and the touch IC 30 determines whether there is a touch by using the level of a sensing signal which is input to the touch sensing unit through a receiving electrode of the touch screen 20.
In the LCD device, the common voltage Vcom is applied to the panel for driving the panel. In this case, the common voltage Vcom passes through common electrodes and common lines of the panel, at which point noise is added into the common voltage by an interaction between elements (gate line, data line, pixel electrode, etc.) disposed in the panel.
When the touch IC 30 is determining whether there is a touch by using a driving signal and a sensing signal, the sensing signal overlaps the common voltage including noise (hereinafter simply referred to as a noise common voltage). For this reason, the sensing signal input to the touch sensing unit of the touch IC 30 inevitably includes noise unlike the driving signal (Tx signal) output from the touch driver 10. Therefore, the driving signal and the sensing have different waveforms. That is, as shown in FIG. 1B, due to the unstable noise common voltage, the sensing signal (Rx signal) received through the receiving electrode is input to the touch IC 30 with the sensing signal including noise.
The touch sensing performance of the touch IC 30 can be degraded by the sensing signal (Rx signal) including noise which is input to the touch IC 30. That is, since a signal affected by noise occurring in the panel is added into the sensing signal (Rx signal) input to the touch IC 30, immunity of the touch IC 30 to noise is reduced in sensing a touch. For this reason, a touch sensitivity of the touch IC 30 can be reduced, and, the reduction in the touch sensitivity greatly affects the touch sensing of the touch IC 30, whereby a desired sensing result cannot be obtained.
To overcome the above-described limitations, technology is developed in which a feedback compensation signal for counteracting the noise common voltage is applied to the touch IC 30, and thus, the touch IC 30 removes the noise from a sensing signal.
However, in the above-described related art, since a new function is added to the touch IC 30, the structure of the touch IC 30 is complicated.
Moreover, in the related art, since a new function and structure should be added to the touch IC 30, the costs of the touch IC and LCD device increase.